1. Field of the Invention
This invention relates to an electrical distribution track in which multiple electrically-isolated, conductive busbars are housed in an elongated enclosure for feeding electricity to take-off devices that may be inserted into the track at any point along the length of the track to make electrical contact with the busbars.
2. Description of the Prior Art
It is common in factories, shops, offices and other buildings to provide overhead electrical power distribution tracks for providing a convenient source of electricity for lights, machines and other electrical devices in the buildings.
Electrical power distribution tracks are typically comprised of an elongated housing containing multiple electrically-isolated, conductive busbars. Track lighting and continuous plug-in busway are typical of this type of track system. Sections of the track can be joined together to form long runs for power distribution. Take-off devices are used to tap power from the track to the load apparatus. The load may be anything from a lamp to a three phase electrical machine. It is desirable to be able to insert take-off devices into or remove them from the track at any point along the track itself and make a secure electrical contact with the busbars.
It is also desirable that the electrical connection between take-off devices and the busbar not require bolts, crimps or other fastening hardware. A pressure connection is easily made or removed and is therefore the method of choice for most busbar to take-off device connections. However, as the ampere rating of the take-off device increases, it is necessary to increase both the contact area and pressure of the connection. Conventional systems are typically limited primarily in the contact area of the connection.
Some known busway systems use a rectangular shaped busbar with periodic sections where a take-off device may be inserted. The take-off devices have one or more stabs per busbar, each stab having at least two members that are spring loaded. When the device is inserted into the busway, these stabs press against the busbar to make the electrical connection. Take-off devices for these systems tend to be complex and expensive. These systems permit take-off devices to be installed only at predetermined locations.
It is also known to use distribution systems that include a formed busbar made of a spring-like material that exerts pressure on the stab of a take-off device when inserted. However, these systems are typically designed such that the contact surface area is a linear point contact along the width of the stab. Because of this, the contact surface is extremely small and can carry only limited current. Systems of this type are typically rated to a maximum of 60 amperes.
It is also known to use a "U" channel busbar in distribution systems, but such systems usually have limited current capacity. To increase the current capacity, copper must be added to the busbar profile, and that makes the profile more rigid and may no longer exert pressure on the contact. In addition, this U-shape does not readily conform to the shape of the stab. If the dimensional fit between the stab and the busbar is not precise, a point contact results as is described above.
None of these prior art systems are adaptable to provide both continuous access by take-off devices and high current capacity.
Typical busway sections are comprised of busbars and insulating support or supports which hold the busbars within an electrically conductive enclosure or housing. Sections can be as long as 20 feet. When the section is assembled, it is desirable that the busbars are fixed rigidly to the insulating support, and the insulating support is fixed rigidly to the enclosure. In many busway systems, the busbar slides into a slot in a continuous elongated insulating support.
There are several known techniques for retaining the busbar rigidly within its insulating support. These include a friction fit in which the busbar is pressed into its insulating support and is held in place by virtue of very tightly fitting components. This is a good method only when the busbar can be coextruded with a continuous insulator support. It is also useful when the busbar is supported at discrete locations by molded plastic components which snap around the busbar. This technique does not lend itself to systems where a continuous insulating support is desired and the busbars cannot be coextruded with the insulator. In a practical sense, only small rectangular profile busbars can be extruded.
Another technique is to fasten busbars to the insulating support by some fastening means such as a pin. Once the busbars have been positioned in the insulator, a hole is drilled through each busbar and the insulator, and then a pin is inserted which fixes the position of the busbars. While this method works, it is very labor intensive to manufacture.
At least one busway system is known in which the busbars are not fixed rigidly in place. The weakness in this approach is that the installer of such a system is responsible to position both the busbars within the insulator and the insulator within the enclosure. Positioning of these components is very important in maintaining safe separation between live electrical parts. A manufacturer cannot be assured that an installer will properly execute this procedure.
All prior art systems have significant drawbacks. The friction fit would not work in many systems because the busbar cannot be coextruded with the insulator in such systems. Fasteners work, but are very labor intensive. There is a need for a simple system to solve the problem.